In a stirred laboratory reactor a biofilm was grown on a gas-permeable silicone membrane, Xylene was added to the bulk fluid as the only carbon source and oxygen was supplied through the membrane. For 66 days the reactor was operated under varying experimental conditions. The concentrations of five components in the reactor effluent and of two in the gas outlet were measured almost daily, the biofilm thickness was determined several times. The measured time series were used to calibrate a mathematical model which calculates pH, aerobic degradation of xylene, and microbial growth, decay and conversion to inert particulate material. The calibrated model was then used to analyze the spatial distribution and dynamics of the xylene-degrading heterotrophic population in the biofilm. The analysis revealed that as long as oxygen was supplied as air heterotrophic cells only grew within a thin zone of about two hundred micrometers near the membrane. Upon the change from air to pure oxygen this zone shifted instantaneously by half of the biofilm thickness of about two millimeters towards the biofilm surface. The immediate ability of the biofilm to convert xylene in a zone which so far had been strictly anaerobic, indicates the presence of a heterotrophic population in this zone. The conclusion is that this population had been transported from the aerobic to the anaerobic zone and that transport and growth of microbial cells are processes which are equally important in biofilms.

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